System and Method for MIMO Split-Physical Layer Scheme for a Wireless Network

ABSTRACT

A system employing split-PHY functionality across multiple access heads to establish multiple paths between a client device (CD) and an access network. Space-time coding includes transmitting one or multiple spatial streams through each access head. Macro transmit diversity includes transmitting a same signal, consisting of one or more spatial streams, from one or more access heads. A CD uses a strongest signal transmitted from at least one access head or diversity combining of two or more signals to recover the signal. Space-time coding includes receiving one or multiple spatial streams through each access head. Macro receive diversity includes receiving a same signal from a CD at one or more access heads. A strongest received signal is used to recover a signal or diversity combining of two or more signals received at different access heads is used to recover the transmitted signal.

FIELD OF THE INVENTION

The invention relates to a split physical (PHY) layer scheme for amultiple input multiple output (MIMO)-based wireless network.

BACKGROUND OF THE INVENTION

Providing a cost effective, robust communications infrastructure usingwireless local area networking (WLAN) technologies is a challenge inenterprise office environments. For example, the area to be coveredwithin the office environment is diverse in size and shape (e.g.corridors, stairwells, atriums, conference rooms, workstation cubicles,etc.). The transmission path between a WLAN-enabled client device and aWLAN access point may be obscured by the building infrastructure (e.g.walls, support columns, stairways), by office equipment (e.g. bookcases, storage units, cubicle partitions, etc.), and by the officeworkers themselves. In addition, the transmission path may be affectedby other equipment operating in the area (e.g. microwave ovens, cellularphones, other WLAN devices, etc.) producing interference that disruptscommunications.

Some elements of the environment are fairly static (e.g. the supportingwalls and columns of the building structure) and can be dealt with tosome degree during initial planning and deployment of the WLAN system.Office equipment, decorations, banners and other furnishings come and goover the lifetime of the WLAN installation and can have an effect onWLAN operation that cannot be anticipated during the planning stages.Interference, whether intentional or unintentional, cannot beanticipated and must be dealt with as it occurs.

Unanticipated changes in the environment may be dealt with either bymodifying the operating parameters of a transmission (e.g. themodulation scheme, the channel frequency), by adjusting the coveragepattern (e.g. through beamforming), or by finding an alternate pathbetween the transmitter and receiver.

Traditionally, finding an alternate path in a WLAN environment meansthat the client device must look for another access point within rangeof its transmission. Normal operation of applications running on theclient device may be disrupted during the time that it takes for theclient device to find a new access point and to establish a connectionthrough that access point.

MIMO technology—recently introduced into the 802.11 WLAN and into anumber of cellular communications standards—provides another means forestablishing multiple paths between an access point and a client device.

SUMMARY OF THE INVENTION

According to a first broad aspect of the invention, there is provided amethod for use in a MIMO-based wireless network, the method comprising:in a centralized manner, performing at least some physical (PHY) layerfunctionality; in a distributed manner, performing at least some PHYlayer functionality, which is distinct from the at least some physicalPHY layer functionality performed in a centralized manner; wherein:performing at least some PHY layer functionality in a centralized mannercomprises performing MIMO space-time coding/decoding; and performing atleast some PHY layer functionality in a distributed manner comprisesperforming analogue modulation/demodulation.

In some embodiments, performing at least some PHY layer functionality ina centralized manner comprises at least one of: performing channelcoding/decoding; performing digital modulation/demodulation; andperforming constellation mapping/de-mapping.

In some embodiments the method further comprises: in a centralizedmanner, performing at least some media access control (MAC) layerfunctionality.

In some embodiments the method further comprises, in a centralizedmanner, performing at least one of: scheduling of transmissions; andselecting at least one technique from a set of techniques for processinga transmit/receive signal.

In some embodiments, performing analogue modulation/demodulationcomprises performing orthogonal frequency divisional modulation (OFDM)modulation/demodulation.

In some embodiments the method further comprises in a centralizedmanner, providing synchronization signals to control the timing ofwireless transmissions.

In some embodiments, performing at least some PHY layer functionality ina centralized manner comprises performing processing of a transmitsignal according to at least one of the following techniques: a)macro-transmit diversity; b) spatial reuse; and c) spatial multiplexing.

In some embodiments, performing at least some PHY layer functionality ina centralized manner comprises performing processing of a receive signalaccording to at least one of the following techniques: a) macro-receivediversity; b) spatial reuse; and c) spatial multiplexing.

In some embodiments, performing at least some PHY layer functionality ina centralized manner is performed by a central radio controller andperforming at least some PHY layer functionality in a distibuted manneris performed by a plurality of remote access heads, each remote accesshead having a coverage pattern, the method further comprising: providingan overall coverage pattern formed collectively by the coverage patternsof each of the plurality of remote access heads that is configured forone or more of: tailoring the wireless coverage to the localized areaprovided by the plurality of remote access heads to a desired size andshape; providing more uniform SINR across containment region; minimizingundesirable signal leakage to areas outside of the containment region;dynamically selecting a subset of the plurality of remote access headsduring each transmission opportunity in order to minimize frame errorrate and/or to maximize throughput.

According to a second broad aspect of the invention, there is provided aMIMO-based wireless network system comprising at least one radiocluster, the radio cluster comprising: a radio controller configured toperform at least some physical (PHY) layer functionality in acentralized manner; and a plurality of remote access heads configured tobe connected to the radio controller and wherein each remote access headis configured to perform at least some PHY layer functionality distinctfrom that which the radio controller is configured to perform, theplurality of remote access heads collectively providing coverage to alocalized area; wherein: the at least some PHY layer functionalityperformed by the radio controller comprises MIMO space-timecoding/decoding; and the at least some PHY layer functionality performedby one or more of the plurality of remote access heads comprisesanalogue modulation/demodulation.

In some embodiments, the at least some PHY layer functionality that theradio controller is configured to perform comprises at least one of:channel coding/decoding; digital modulation/demodulation; andconstellation mapping/de-mapping.

In some embodiments, the radio controller is configured to perform, in acentralized manner, one or more of: scheduling transmission and receiptof packets; and selecting at least one technique from a set oftechniques for processing a transmit/receive packet.

In some embodiments, the analogue modulation/demodulation comprisesorthogonal frequency divisional modulation (OFDM)modulation/demodulation.

In some embodiments, the radio controller communicates with theplurality of remote access heads using wired or unwired network linkswherein: wired network link protocols include one or more of Ethernet,Infiniband, frame relay, and ATM; and unwired network link protocolsinclude one or more of IEEE 802.11, IEEE 802.16, UWB, and point-to-pointmicrowave.

In some embodiments, the radio controller is configured to providesynchronization signals to the plurality of remote access heads tocontrol the timing of wireless transmissions.

In some embodiments, for communications transmitted to or received fromthe plurality of remote access heads, the radio controller is configuredto perform processing of a transmission or received signal according toat least one of the following techniques: a) macro-transmit diversity;b) macro-receive diversity; c) spatial reuse for one or both of atransmit signal and a receive signal; d) spatial multiplexing for one orboth of a transmit signal and a receive signal; and e) some combinationthereof.

In some embodiments, the MIMO-based wireless network system isconfigured to operate in any one of: a wireless local area network; awide area cellular network; and a wireless room-area network.

In some embodiments, for a given radio cluster, each remote access headof the plurality of remote access heads is connected to the radiocontroller: directly via a point-to-point connection; or indirectly viaa fan-out arrangement through one or more other remote access heads.

In some embodiments, the at least one radio cluster operates as a singlefrequency network.

In some embodiments, when the system includes more than one radiocluster and each radio cluster operates as a single frequency network,the system utilizes a frequency plan in allocating which radio clustersuse which frequencies.

In some embodiments, the system further comprising a network linkbetween the radio controller and each of the plurality of remote accessheads, the network link configured to provide one or more of: facilitiesto transmit packets between the radio controller and each remote accesshead of the plurality of remote access heads; synchronization signals tobe transmitted by the radio controller to each remote access head of theplurality of remote access heads; and electrical power to be provided bythe radio controller to at least one remote access head of the pluralityof remote access heads.

Some embodiments of the invention address the need for a more robustwireless infrastructure in enterprise office deployments and in widearea cellular network deployments.

Other aspects and features of the present invention will become apparentto those ordinarily skilled in the art upon review of the followingdescription of specific embodiments of the invention in conjunction withthe accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described with reference to theattached drawings in which:

FIG. 1 is a schematic diagram of a conventional single access pointproviding coverage for a given area;

FIG. 2 is a schematic diagram of a group of access heads providingcoverage for a given area in a distributed manner according to anembodiment of the invention;

FIG. 3 is a schematic diagram of a radio cluster architecture includinga radio controller and multiple access heads according to an embodimentof the invention;

FIG. 4 is a block diagram of a wireless local area network (WLAN)topology having a single radio cluster illustrating MAC and PHYfunctional partitioning according to an embodiment of the invention;

FIG. 5A is a flowchart illustrating a method for providing MIMOsplit-physical (PHY) layer functionality according to an embodiment ofthe invention;

FIG. 5B is a flowchart illustrating an example of a method for providingMIMO split-PHY layer functionality according to another embodiment ofthe invention;

FIG. 5C is a flowchart illustrating an example of a method for providingMIMO split-physical (PHY) layer functionality according to a furtherembodiment of the invention;

FIGS. 6A and 6B are schematic diagrams illustrating examples ofcustomized radio cluster antenna pattern layouts for specifically shapedcoverage areas;

FIG. 7 is a block diagram of a wireless local area network (WLAN)topology having multiple radio clusters illustrating MAC and PHYfunctional partitioning according to an embodiment of the invention;

FIG. 8 is a block diagram of transmitter and receiver chains in theradio cluster according to an embodiment of the invention;

FIG. 9 is a schematic diagram of downlink transmission in an exampleradio cluster including a radio controller, multiple access heads andmultiple client devices; and

FIG. 10 is a schematic diagram of uplink transmission in an exampleradio cluster including a radio controller, multiple access heads andmultiple client devices.

DETAILED DESCRIPTION OF THE EMBODIMENTS OF THE INVENTION

Reliability and consistent coverage are often cited as problems thatplague current wireless network installations. Such problems can havemany causes. For example, holes in a coverage area due to obstructionsbetween a client device and a network access point. This is a particularproblem when furniture, equipment, etc. is installed or moved after aninitial installation of wireless network access points. Another cause istransient shadowing effects due to movement of people and objects withinthe coverage area. A further cause may be path loss due to the locationof the client at some distance from the nearest wireless network accesspoint, also known as “cell edge effect”. Yet another cause may beinterference, mostly unintentional, caused by other equipment operatingon the same or an adjacent frequency close to the client device and/orthe wireless network access point.

As an alternative to a conventional radio subsystem of a traditionalwireless network Access Point (AP), embodiments of the invention providea distributed radio cluster consisting of a central radio controller anda plurality of remote access heads. The central radio controllerperforms some PHY layer processing and provides a stream of space-timesymbols to one or more of the plurality of remote access heads. Eachremote access head performs additional PHY layer processing, distinctfrom that performed by the central radio controller. Each remote accesshead includes one or more antennas and hardware and/or software toenable transmission and receipt of signals to/from client devices andto/from the radio controller. While the access heads are described asbeing “remote” from the central radio controller, it is to be understoodthat at least one access head could be collocated with the central radiocontroller.

The present application describes a system and method for dynamicallyemploying MIMO and macro diversity across multiple access heads toestablish multiple paths between a client device (CD) and an accessnetwork in order to address one or more of the above described coverageissues. MIMO space-time coding may include transmitting one space-timestream or multiple space-time streams through each access head. Macrotransmit diversity may include transmitting a same signal, consisting ofone or more space-time streams, from one or more access heads such thata CD can use a strongest signal of the signals transmitted from at leastone access head or diversity combining of two or more of the transmittedsignals to recover the transmitted signal at the CD. MIMO space-timecoding may include receiving one space-time stream or multiplespace-time streams through each access head. Macro receive diversity mayinclude receiving a same signal from a CD at one or more access headssuch that a strongest signal of the signals received at one or moreaccess heads can be used to recover a signal or diversity combining oftwo or more of signals received at different respective access heads canbe used to recover the transmitted signal.

In addition, through dynamic spatial reuse and/or spatial multiplexingof traffic across multiple access heads, embodiments of this inventionmay provide better spectrum management and may increase the throughputand/or capacity of the overall system by transmitting to, or receivingfrom, several CDs in parallel.

Embodiments of the present invention employ macro diversity acrossmultiple access heads to aid in improving reliability and mitigatecoverage problems in wireless network installations.

Embodiments of the invention are generally described here with respectto a wireless local area network. However, this is not intended to limitthe invention. In some embodiments, the methods, devices and systemsdescribed herein may be applied to wide area cellular networks and/orwireless room-area networks.

FIG. 1 illustrates a conventional wireless network AP 100 having twoantennas, for micro receive diversity, providing coverage 105 in adefined area 110 from a central location in the defined area. A firstclient device (CD) 120 is shown at the edge of the area of coverage 105and a second CD 130 is shown within the area of coverage 105. Anobstacle 140 to a signal being received by CD 130 is also shown inFIG. 1. Providing coverage from a central location may result indegradation of the signal quality for a CD that is further away from theAP, such as with CD 120, and can result in loss of signal if there areobstructions between the AP and the target CD, such as with obstacle 140and CD 130.

Referring now to FIG. 2, an exemplary arrangement according to an aspectof the present invention will be described. The arrangement provides aradio cluster configured to replace the centralized one or more antennasof the AP used in the conventional wireless network AP of FIG. 1. Theradio cluster in this example includes six access heads (AH) 200 a-200 fthat are distributed throughout a defined area 210 and a radiocontroller (not shown) connected to all of the access heads. The AHs 200a-200 f each have a respective coverage area 205 a-205 f. As can be seenfrom FIG. 2, there is overlap of coverage areas of adjacent accessheads.

While six access heads are illustrated in the example of FIG. 2, thenumber of access heads is implementation specific. The number of accessheads may depend on one or more factors such as, but not limited to, thearea of coverage, obstacles within the coverage area and by the desiredperformance metrics of the radio cluster (e.g. SINR and/or throughputtargets).

FIG. 2 also illustrates a first CD 220 shown within overlapping coverageareas 205 b, 205 d and 205 e. A second CD 230 is shown withinoverlapping coverage areas 205 e and 205 f. A CD may be, but is notlimited to, such devices as cellular telephone, wireless enabledcomputers, and wireless enabled personal data assistants (PDAs).

An obstacle 240 is also shown in FIG. 2 located between AH 200 e and CD230. Obstacle 240 may obstruct a signal transmitted by access head 200 eintended for CD 230, but AH 200 f still has an unobstructed, direct lineof sight path to CD 230. Having multiple access heads configured tocollectively provide coverage for a define region, such that one or moremay transmit to/receive from a CD in this manner may allow improvedsignal reception over the case illustrated in FIG. 1, in which obstacle140 obstructs a transmit signal in the direction of CD 130 from the onlyavailable signal source AP 100.

In a downlink transmission scenario (transmission sent from one or moreaccess heads toward at least one CD) in which the radio clustercommunicates with CD 220, the radio controller determines which accessheads are able to communicate with CD 220. As access heads 200 b,200d,200 e are able to communicate with CD 220, the radio controllerselects those particular access heads to transmit to CD 220. CD 220receives signals from access heads 200 b,200 d,200 e and either combinesthe signals received from two or more of the access heads using someform of diversity combining or uses only a single strongest signalreceived from one of the access heads. Contemporaneously, the radiocontroller may select access head 200 f to transmit to CD 230 sinceobstacle 240 prevents access head 200 e from communicating with CD 230.

On the downlink, the radio controller may instruct one or more accessheads to simultaneously transmit information towards one or more CDs. Atevery downlink transmission opportunity, the radio controller may selectfrom amongst a number of performance-enhancing techniques for performingtransmit signal processing.

A first technique is macro-transmit diversity in which redundantinformation is transmitted by multiple access heads to a single CD.Redundant transmission of information may reduce the frame error rate(FER) seen by the CD thereby improving the quality of a radio linkbetween one or more access head and CD.

A second technique is spatial reuse in which information is transmittedto different CDs by different access heads simultaneously within a givencoverage area. The CDs and corresponding access heads to transmit to theCDs are selected so that the transmissions do not interfere with eachother, thereby increasing the overall throughput of the system.

A third technique is spatial multiplexing in which multiple spatialstreams are formed to transmit information to one CD or to multiple CDsby access heads with overlapping coverage. Allocating multiple spatialstreams to one CD will increase the throughput experienced by that CD;allocating different streams to different CDs will increase the overallthroughput of the system.

The various techniques described above may also be combined—for example,a given spatial stream may be scheduled for transmission by more thanone access head, allowing the combined benefits of spatial multiplexingand transmit diversity to be realised, which has the effect of, in agiven coverage area, transmitting to multiple CDs simultaneously frommultiple AHs.

In some embodiments, an access head may have more than one antenna. Theradio controller may provide different streams for transmission by oneor more antennas of the access head. This may be performed by the radiocontroller sending streams addressed to individual antennas of theaccess head. One manner of considering such a scenario is that eachgroup of one or more antennas transmitting a different stream than otherantennas in the access head form a physically separate access head, interms of the hardware/software used to implement the processing in theaccess head for each different stream. That is the hardware/software ofmultiple access heads could be packaged in a single physical entity andone or more of the access heads used as appropriate depending how manystreams are received at the access head. Another manner of consideringsuch a scenario is that each group of one or more antennas transmittinga different stream than other antennas in the access head is considereda virtual access head. That is the hardware/software of multiple accessheads may be shared for processing the different streams for the accesshead.

In an uplink transmission scenario (transmissions received at one ormore access head from at least one CD) in which CD 220 communicates withone or more access head of the radio cluster, access heads 200 b,200d,200 e are able to communicate with CD 220 and receive a signal from CD220. The radio controller connected to access heads 200 b,200 d,200 ecan either combine the signals received by two or more of the accessheads using some form of diversity combining or use only a singlestrongest signal received by one of the access heads. Contemporaneously,the radio controller may allow CD 230 to transmit to access head 200 fsince obstacle 240 prevents access head 200 e from communicating with CD230.

On the uplink, a transmission from a CD may be received by one or moreaccess heads. The received information is forwarded to the radiocontroller by each access head. At every uplink transmissionopportunity, the radio controller may select from amongst a number ofperformance-enhancing techniques for processing the received signal.

A first technique is macro-receive diversity in which informationreceived by the radio controller from multiple access heads is combined.For example, the information may be combined using diversity combiningtechniques. Combining received signals from multiple access heads mayreduce the frame error rate seen by the radio controller, therebyimproving the quality of the radio link between the CD and the radiocontroller.

A second technique is spatial reuse in which information is receivedfrom different CDs by different access heads. The CDs and correspondingaccess heads that will receive from the CDs are selected so that thetransmissions from the CDs do not interfere with each other, therebyincreasing the overall throughput of the system.

A third technique is spatial multiplexing in which the spatial streamstransmitted simultaneously by different CDs are coded to distinguishthemselves when received by access heads with overlapping coveragethereby increasing the overall throughput of the system.

While only two CDs are illustrated in the example of FIG. 2, it is to beunderstood that any number of CDs may be operating within the areadefined by the coverage of the access heads. The number of CDs withinthe coverage area of the access heads will vary as CDs migrate in andout of the coverage area. As a CD migrates within the coverage area, theradio controller may determine that different groups of one or moreaccess heads will transmit to or receive from the CD.

From the perspective of a CD, the one or more of the plurality of accessheads transmitting to or receiving from the CD operate collectively asthough they were a single access point. In some embodiments, thedistance between a given CD and a nearest access head is reduced ascompared to the case of only a single Access Point used to cover anentire area. The reduced distance may improve signal quality by reducingpropagation losses.

In some embodiments, neighbouring access heads may provide overlappingcoverage, thereby providing alternate transmission paths to avoidobstacles. An example of this is access heads 200 b,200 d,200 e in FIG.2 providing overlapping coverage at the location of CD 220.

A decision on which set of access heads to use for communicating withthe CD may be made by the radio controller as frequently as at everydownlink and uplink transmission opportunity. The CD is unaware of thedecisions made by the controller, as the CD sees the entire cluster as asingle AP, so there is no disruption in the operation of applications onthe CD. The controller is able to use its selection of access heads toadapt to movement of the CD and/or to conditions in the RF environment,which may experience rapidly changing conditions, in order to improveoverall performance and capacity.

With reference to FIG. 3 a more detailed example of a radio cluster willnow be described. Radio cluster 300 includes a radio controller 310 andten access heads 320 a-320 j. The radio controller 310 is connected tothe access heads 320 a-320 j via network links. An access head may beconnected to the radio controller directly via a point-to-point networklink. In the illustrated example, this is represented by network linksbetween radio controller 310 and access heads 320 a, 320 d and 320 h,respectively. Alternatively, an access head may be connected to theradio controller indirectly via a fan-out arrangement through anotheraccess head. In the illustrated example, this is represented by anetwork link connecting radio controller 310 to access head 320 a andnetwork links between access head 320 a and access head 320 b and 320 c,respectively, a network link connecting radio controller 310 to accesshead 320 d and network links between access head 320 d and access head320 e, 320 f and 320 g, respectively, and a network link connectingradio controller 310 to access head 320 h and network links betweenaccess head 320 h and access head 320 i and 320 j, respectively. In theillustrated example three access heads are shown connected directlypoint-to-point and seven connected by fan-out, but it is to beunderstood that such connections are implementation specific and willvary based on the desired configuration for a given coverage area.

The type of network link is implementation specific. In someembodiments, the radio controller 310 is connected to the access heads320 a-320 j via standard data communications grade cables. A particularexample of such a cable in an enterprise office environment is Category5 unshielded twisted pair.

In some embodiments, the network link carries information in packets orframes, for example Ethernet frames that are exchanged between the radiocontroller and the access heads. In some embodiments, the network linkcarries timing synchronization signals between the radio controller 310and the access heads. In some embodiments, the network link provideselectrical power for the access heads. In some embodiments, the accessheads are powered in an alternative manner than by using a power signalfrom the network link, for example the access heads may be plugged in toan electrical socket local to their placement.

In some implementations, the radio controller co-ordinates transmissionof radio link packets and/or frames on the downlink. Transmission to oneor more CDs may be via one or more access heads in the cluster, asdetermined by the radio controller. In some implementations, the radiocontroller correlates communication packets and/or frames received onthe uplink. Reception from each CD may be via one or more access headsin the cluster, as determined by the radio controller.

There is a single radio controller per radio cluster in a non-redundantconfiguration. In some embodiments, more than one radio controller isincluded for the radio cluster to provide desired redundancy.

An access head contains one or more antennas. In FIG. 3 access heads 320a, 320 g and 320 h are shown to have two antennas and other access headsare shown to have a single antenna. The number of antennas per accesshead is implementation specific.

The access heads contain hardware/circuitry for transmitting andreceiving signals. In some implementations this includes, but is notlimited to, circuitry to convert between an analogue signal passed toand from a CD over a wireless radio link and a digital signal passed toand from the radio controller over the wireless network infrastructure.

FIG. 3 illustrates an example of a single radio cluster 300. In someembodiments, more than one radio cluster can be included in a system inwhich each radio cluster is arranged to cover a specific area. Forexample, a system having multiple radio clusters may be a multi-flooroffice building in which each floor of the office building has it ownradio cluster. Some floors may even be subdivided such that multipleradio clusters operate on one or more of the floors.

In some embodiments, the radio cluster operates as a single frequencynetwork (SFN). In an SFN every access head within a given radio clustertransmits and receives information on the same RF channel. In someembodiments, different radio clusters operate on different RF channelsaccording to a desired frequency plan, in order to minimise interferencebetween radio clusters.

FIG. 4 illustrates an enterprise network topology 340 having a singleradio cluster 350. The radio cluster 350 includes a radio controller 352and a plurality of access heads 354. The network links between the radiocontroller and plurality of access heads in the radio cluster 350 areillustrated to be Ethernet based links 356. The use of Ethernet basedlinks as shown in FIG. 4 is for example purposes and is not intended tolimit the type of network links that could be used. In alternativeembodiments, for example in an office environment, the network links maybe a high speed wired link like Infiniband, UWB or point-to-pointmicrowave. In a wide area environment, it may be frame relay or ATM or(metro) Ethernet, possibly over microwave facilities.

In FIG. 4, the radio cluster 350 is connected to a WLAN switch 360 via abridged Ethernet LAN 370. The WLAN switch 360 interfaces with a largeroverall network 380 allowing the forwarding of packets to and from theradio cluster that is currently serving a given CD. In the illustratedexample the network is an enterprise network.

Some embodiments of the invention partition functionality defined in thephysical (PHY) layer of a MIMO-based product across multiple networkelements. In some embodiments of the present invention, responsibilityfor the PHY functionality within a radio cluster is split between theradio controller 352 and each of the access heads of the plurality ofaccess heads 354. This is indicated in FIG. 4 as “Split PHY”. Within theradio cluster 350, the radio controller 352 implements packet data unit(PDU) coding and modulation scheme (e.g. QPSK, QAM) functionality, aswell as MIMO space-time coding functionality. The plurality of accessheads 354 implement PHY modulator and demodulator functionality, such asthat based on OFDM (orthogonal frequency division multiplexing), as wellas antenna subsystem functionality for the radio cluster.

In some embodiments of the invention, a split-MAC (media access control)architecture is implemented. In a split-MAC architecture, MACfunctionality is split between the WLAN switch 360 and the radiocontroller 352 within the radio clusters 340. This is indicated in FIG.4 as “Split MAC”.

In the split-MAC arrangement, the WLAN switch 360 implements theso-called “Upper MAC” functionality. The “Upper MAC” functionality is,such as, but not limited to, management and control procedures of theMAC protocol stack. The radio controller 352 implements “Lower MAC”functionality, such as, but not limited to, scheduling, access control,automatic repeat requests, etc.

A method for implementing a MIMO split-PHY architecture will now bedescribed with reference to FIG. 5A. FIG. 5A illustrates a flow chartfor a method for use in a MIMO-based wireless network. A first step 5-1of the method includes in a centralized manner, performing at least somephysical (PHY) layer functionality including MIMO space-timecoding/decoding. A second step 5-2 involves in a distributed manner,performing at least some PHY layer functionality, which is distinct fromthe at least some physical PHY layer functionality performed in acentralized manner, including analogue modulation/demodulation. Theplurality of remote locations collectively provide wireless coverage toa localized area.

In some embodiments, performing at least some PHY layer functionality ina centralized manner involves performing channel coding/decoding. Insome embodiments, performing at least some PHY layer functionality in acentralized manner involves performing digital modulation/demodulationand constellation mapping/de-mapping.

In some embodiments, performing at least some PHY layer functionality ina centralized manner involves performing processing of a transmit signalaccording to at least one of the following techniques: a) macro transmitdiversity; b) spatial reuse; and c) spatial multiplexing.

In some embodiments, performing at least some PHY layer functionality ina centralized manner involves performing processing of a receive signalaccording to at least one of the following techniques: a) macro receivediversity; b) spatial reuse; and c) spatial multiplexing.

In some embodiments the method further involves in a centralized manner,providing synchronization signals to control the timing of wirelesstransmissions.

In some embodiments, performing analogue modulation/demodulationinvolves performing orthogonal frequency divisional modulation (OFDM)modulation/demodulation.

In some embodiments, the method further involves performing in acentralized manner at least some media access control (MAC) layerfunctionality. In some embodiments, performing at least some MAC layerfunctionality in a centralized manner involves performing scheduling oftransmissions, controlling access and controlling automatic repeatrequests.

A detailed example of a method for implementing a MIMO split-PHYarchitecture will now be described with reference to FIG. 5B. FIG. 5Billustrates a flow chart for a method for downlink communication in aMIMO-based wireless network. For downlink communication from the radiocontroller via one or more access heads to a CD initial steps areperformed by the radio controller. A first step 5-10 involves the radiocontroller receiving at least one media access control protocol dataunit (MAC PDU) for transmission to at least one CD. A subsequent step5-12 involves generating at least one stream of space-time symbols fromthe at least one MAC PDU. Generating the at least one stream includesspace-time coding and may include performing other processing steps,such as, for example, channel encoding and/or digital modulation. Afurther step 5-14 performed by the radio controller is providing the atleast one stream of space-time symbols to one or more remote accessheads. A further step performed by the radio controller may includeselecting which techniques i.e. macro-transmit diversity, spatial reuse,spatial multiplexing, will be used for processing the at least one MACPDU.

Following the steps performed at the radio controller, the access headsperform additional steps prior to transmitting the signal to at leastone CD. A first step 5-16 performed at an access head intended toreceive at least one space-time stream of symbols from the radiocontroller involves receiving at least one stream of space-time symbols.After the stream of space-time symbols is received, a further step 5-18involves the access head that received the stream of space-time symbolsmodulating the symbols. In some embodiments, the modulation may be OFDMbased modulation. An additional step 5-20, after the symbols aremodulated, involves the access head transmitting the modulated symbolsfrom one or more access head antennas to the CD for which the originalMAC PDU was intended.

Another detailed example of a method for implementing a MIMO split-PHYarchitecture will now be described with reference to FIG. 5C. FIG. 5Cillustrates a flow chart for a method for uplink communication in aMIMO-based wireless network. For uplink communication from at least oneCD via one or more access heads to the radio controller, initial stepsare performed by the access heads. A first step 5-40 involves, at one ormore of the access heads, receiving a signal on the access head'santennas. A subsequent step 5-42 involves the access head that receivedthe signal demodulating the signal so as to recover a stream ofspace-time symbols. In some embodiments the received signal is an OFDMsignal and the demodulation is OFDM demodulation. A further step 5-44performed by the access head is providing a recovered stream ofspace-time symbols to the radio controller.

Following the steps performed at each of one or more access heads, theradio controller performs additional steps. A first step 5-46 at theradio controller involves receiving streams of space-time symbols fromeach of the access heads. After the streams of symbols are received, afurther step 5-48 involves the radio controller decoding the space-timestreams of symbols. An additional step 5-50, after the symbols arerecovered involves the radio controller reconstructing MAC PDUs from thedecoded space-time streams. Reconstructing the MAC PDUs may involvefurther processing such as channel decoding and/or digital demodulationand/or error correction.

Each access head may also be configured to provide a different coveragepattern in order to tailor the illumination provided by the radiocluster to the size and shape of the containment region. Particularexamples will be described referring to FIGS. 6A and 6B.

In FIG. 6A, a rectangular containment region 400 (e.g. a room) isilluminated from two adjacent corners 410,420 using a first access headAH-1 in one corner 410 and a second access head AH-2 in the other corner420 that each provide a 90 degree coverage pattern, indicated byreference characters 430 and 432, respectively. A third access head AH-3is located on a wall opposite to a wall shared by the two adjacentcorners 410,420. The third access head AH-3 provides a 180 degreecoverage pattern 434 from the middle of the wall. This collection ofgroups may result in more uniform coverage and, hence, more predictableservice by providing overlapping coverage in those areas of thecontainment region that are furthest from an access head. Macrodiversity may also be used to transmit redundant information on two ormore of the AHs and/or receive from each CD on two or more of the AHs,as determined by the radio controller. The selected coverage pattern ofthe three access heads may reduce inter-cluster interference byminimising signal strengths seen outside the desired containment area.

FIG. 6B shows another example of a coverage strategy with a first accesshead AH-P providing 360 degree omnidirectional coverage over a givenregion 450. Six auxiliary access heads AH-a1 to AH-a6 provide fill-incoverage using lower power, 180 degree coverage, indicated by referencecharacters 455,460,465,470,475,480, from the edge of region 450. Thiskind of illumination pattern eliminates many of the problems typicallyarising from propagation loss at the edge of a cell. Spatial reuse gainscan also be realised by, for example, simultaneously transmittingdifferent spatial streams to CDs within the coverage areas of AH-a1,AH-a3 and AH-a5. In such an example, the controller may or may notfurther decide not to transmit anything from AH-a2, AH-a4 and AH-a6 inorder to minimise the interference seen in AH-a1, AH-a3 and AH-a5.

Any of the methods described above in FIG. 5A, 5B or 5C may furtherinvolve providing an overall coverage pattern formed collectively by thecoverage patterns of each of the plurality of remote access heads, theoverall coverage pattern being configured for one or more of: tailoringthe wireless coverage to the localized area provided by the plurality ofremote access heads to a desired size and shape; providing more uniformSINR across containment region; minimizing undesirable signal leakage toareas outside of the containment region; dynamically selecting a subsetof the plurality of remote access heads during each transmission/receiveopportunity in order to minimize frame error rate and/or to maximizethroughput.

A wireless network may be comprised of multiple radio clusters. Anexample of a network topology having multiple radio clusters will bedescribed with regard to FIG. 7.

FIG. 7 is similar to FIG. 4, but includes multiple radio clusters. FIG.7 illustrates a wireless network topology 500 having three radioclusters 510,520,530. Each radio cluster includes a radio controller512,522,532 and a plurality of access heads 514,524,534. The linksbetween the radio controller and plurality of access heads in each radiocluster are illustrated to be Ethernet based network links 516,526,536.The radio clusters 510,520,530 are connected to a WLAN switch 550 via abridged Ethernet LAN 540. The WLAN switch 550 interfaces with a largeroverall network 560 allowing the forwarding of packets to and from agiven radio cluster that is currently serving a given CD. In theillustrated example the overall network is an enterprise network.

Some embodiments of the invention partition functionality defined in thePHY layer of a MIMO-based product across multiple network elements. Insome embodiments of the present invention, responsibility for the PHYfunctionality within a radio cluster is split between the radiocontroller and each of the access heads. Within each radio cluster510,520,530, the radio controllers 512,522,532 implement packet dataunit (PDU) coding and digital modulation scheme (e.g. QPSK, QAM)functionality, as well as MIMO space-time coding functionality. This isindicated in FIG. 7 as “MIMO PHY”. The access heads 514,524,534implement PHY layer modulator and demodulator functionality such as OFDMand antenna subsystem functionality for the radio cluster. This isindicated in FIG. 7 as “OFDM PHY”.

Timing information may be distributed by the radio controller512,522,532 to the remote access heads 514,524,534 using a clocksynchronization method. The clock synchronization method isimplementation specific. A particular example of a clock synchronizationmethod is one that is consistent with IEEE 1588 or with ITU-T G.8261.

All of the access heads within a radio cluster are synchronized tomaximise performance for both downlink and uplink operations. The radiocontroller acts as a master clock source for the radio cluster. Theradio controller clock may be generated by a local electrical circuit orit may be derived from a precision timing signal supplied by an entityelsewhere in the network.

Timing information is periodically sent from the radio controller toeach of the access heads. Each access head recovers this timinginformation to drive its own local clock and to synchronise that clockwith one used by the radio controller. Precision timing information maybe delivered either through packet-based protocols or through physicallayer synchronization. The latter technique is typically used when theaccess heads are directly connected to the radio controller via apoint-to-point network link while the former technique may be used whenan access head is either directly connected via a point-to-point link orconnected indirectly via a fan-out arrangement through another accesshead.

In some embodiments of the invention, a split-MAC (media access control)architecture is implemented. In a WLAN split-MAC architecture, MACfunctionality is split between the WLAN Switch 550 and the radiocontroller 512,522,532 within each of the radio clusters 510,520,530.

In some embodiments, the connection between the radio controller and theaccess heads may use Ethernet bridging protocols. In some embodiments,Ethernet framing procedures are used over a simplified and dedicatedfan-out network. Bridged Ethernet networks often use a number of generalpurpose management and control protocols for topology discovery, servicediscovery, multicast group management, etc. A “simplified and dedicatedfan-out network” is intended to describe a network with the ability toforward Ethernet data frames, but does not necessarily support suchgeneral purpose protocols.

The use of bridged Ethernet networks is an example of a type of networkon which some embodiments of the invention may operate and is notintended to limit the type of network. In alternative embodiments,technologies such as Infiniband, frame relay, ATM and (metro) Ethernetover copper, fibre or microwave may be appropriate.

A further detailed example of a radio cluster including a radiocontroller and access heads will be described with reference to FIG. 8.No CDs are shown in FIG. 8, but their presence is implied.

A transmitter chain 630 and a receiver chain 640 are each formed by arespective collection of elements in the combination of the radiocontroller 610 and a plurality of access heads 620, 670. Only a singleaccess head 620 is shown in detail. The transmitter chain 630 includes achannel encoder 634, a digital modulator 636 and a space-time coder 638in the radio controller 610 and an OFDM modulator 622 in the access head620. The receiver chain 640 includes an OFDM demodulator 624 in theaccess head 620 and a space-time decoder 642, a digital demodulator 644and a channel decoder 646 in the radio controller 610.

The radio controller 610 includes a timing signal generator 650 forproviding timing information to the transmitter chain 630 and to thereceiver chain 640 in the combination of radio controller 610 and accesshead 620, as well as for the transmitter and receiver chains in thecombinations of the radio controller 610 with the other access heads670.

The radio controller 610 includes a scheduler 615 for schedulingdownlink transmission by the combination of the radio controller 610 andrespective access heads 620,670 and for scheduling uplink transmissionby CDs (not shown) within the coverage area of the radio cluster.

Each access head, as will be described with specific reference to accesshead 620, includes a timing signal recoverer 660 configured tocorrespond with the timing signal generator 650 in radio controller 610.The timing signal recoverer 660 is configured to correspond with OFDMmodulator 622 and OFDM demodulator 624 in access head 620. Connected tothe OFDM modulator 622 is an antenna for transmitting to CDs. Connectedto the OFDM demodulator 624 is an antenna for receiving from CDs. Whileonly a single antenna is illustrated, it is to be understood that theremay be one, two or more antennas coupled to each of the OFDM modulator622 and OFDM demodulator 624.

While the illustrated example shows the radio controller 610 having achannel encoder 634, a digital modulator 636, a space-time coder 638, aspace-time decoder 642, a digital demodulator 644 and a channel decoder646, not all of these functionalities may be used in all embodiments ofthe invention for processing a frame/packet for transmission to the CD.

In some embodiments the scheduler or another dedicated function may beresponsible for selecting which functionalities and/orperformance-enhancing techniques, i.e. macro-transmit diversity,macro-receive diversity, spatial reuse, spatial multiplexing, will beused for processing the frame/packet.

Operation of the transmit chain and receive chains will now be describedfor a particular example based on IEEE 802.11n standard with referenceto FIG. 8. It is to be understood that embodiments of the invention maybe applied to networks other than those compliant with IEEE 802.11n.

Transmit Chain Operations (Towards a Client Device)

A MAC protocol data unit (MPDU) 632—an 802.11n control, management ordata frame—is selected for transmission by the scheduler 615 residing inthe radio controller 610. The MPDU 632 may have been received from aWLAN switch or it may have been generated internally by a lower layerMAC function residing in the radio controller 610. The channel encoder634 prepares the MPDU 632 for transmission over a radio channel betweenthe access heads 620,670 and a target CD through the addition of PHYinformation elements (e.g. header) and frame protection elements (e.g.FEC). In some embodiments, preparing the transmission may also includescrambling and/or interleaving the bit stream of the MPDU 632. Aresulting PHY protocol data unit (PPDU) is mapped onto a spatial streamand segmented into a sequence of fixed length bit blocks according to aselected modulation scheme (e.g. 6-bit blocks for 64QAM) by digitalmodulator 636. Also in digital modulator 636, each bit block is mappedonto a constellation point of the modulation scheme and converted to anequivalent complex number.

Based on information provided by one or more access heads 620,670 thatpreviously received information from the target CD, the scheduler 615selects a set of access heads that will be used to transmit to thetarget CD. In some embodiments, the scheduler 615 determines which ofthe performance-enhancing techniques described above, macro transmitdiversity, spatial reuse and spatial multiplexing, are applied duringthe downlink transmission opportunity.

The spatial streams scheduled for contemporaneous transmission by a setof access heads are processed by the space-time coder 638 in the radiocontroller 610 to produce a set of space-time streams. Each space-timestream is sent by the radio controller 610 to one or more access heads620,670 where it is modulated onto OFDM carriers by the OFDM modulator622 and transmitted over the radio link via antenna 626.

A particular example of preparing a transmission in a radio cluster willnow be described with reference to FIG. 9. FIG. 9 illustrates a radiocluster 700 including a radio controller 710 connected to seven accessheads identified as AH-1—AH-7. At a given point in time, three clientdevices CD-A, CD-B and CD-C are within the coverage area of the radiocluster 700. CD-A is within range of, and receives downlinkcommunication from, access heads AH-1, AH-2 and AH-3. CD-B is withinrange of, and receives downlink communication from, access heads AH-2,AH-3 and AH-4. CD-C is within range of, and receives downlinkcommunication from, access heads AH-6 and AH-7. Access head AH-5 iscurrently not communicating with any CDs.

Client Device A (CD-A) is within the coverage areas of AH-1, AH-2 andAH-3, which comprise the serving set of CD-A—i.e.

{CD-A} _(serving)={AH-1,AH-2,AH-3}.

Similarly for CD-B and CD-C:

{CD-B} _(serving)={AH-2,AH-3,AH-4} and

{CD-C} _(serving)={AH-6,AH-7}.

At the start of a downlink transmission opportunity, a scheduler in theradio controller 710 determines that {CD-C}_(serving) does not overlapwith {CD-A}_(serving) or with {CD-B}_(serving) thereby allowingtransmissions to CD-C to occur without interference from other accessheads and providing spatial reuse. Therefore, a transmission set, thatis the set of access heads that will transmit to a CD, for CD-C is setequal to the serving set of CD-C:

{CD-C} _(Tx)={CD-C} _(serving)={AH-6,AH-7}.

Since the serving sets of CD-A and CD-B are overlapping, the servingsets of CD-A and CD-B are combined to form a transmission set:

{CD-A+CD-B} _(TX)={AH-1,AH-2,AH-3,AH-4}.

The scheduler of the radio controller can now treat transmission set{CD-A+CD-B}_(TX) as a 4-branch MIMO transmission chain, i.e. the radiocontroller and the four transmit access heads AH-1, AH-2, AH-3, AH-4,with two spatial streams, i.e. one stream for each of CD-A and CD-B. Thescheduler treats transmission set {CD-C}_(Tx) as a 2-branch MIMOtransmission chain i.e. the radio controller and the two transmit accessheads AH-6, AH-7, with one spatial stream i.e. the stream for CD-C.

After mapping onto constellation points of the selected modulationscheme, a spatial stream consists of a sequence of 2N (N>0) complexnumbers. For CD-A, this is represented as a spatial stream sequence:

[CD-A] _(SS) =[a ₁ ,a ₂ ,a ₃ ,a ₄ , . . . a _(i) ,a _(i+1), . . . ]where a _(i) is a complex number.

Similarly for CD-B and CD-C:

[CD-B] _(SS) =[b ₁ ,b ₂ ,b ₃ ,b ₄ ,b _(i) ,b _(i+1), . . . ] and

[CD-C] _(SS) =[c ₁ ,c ₂ ,c ₃ ,c ₄ , . . . ,c _(i) ,c _(i+1), . . . ].

Applying a known space-time block coding algorithm to two spatialstreams, [CD-A]_(SS) and [CD-B]_(SS), across four space-time streams,i.e. the signals to be transmitted from the four access heads of thetransmission set {CD-A+CD-B}_(Tx), yields the following sequence ofcomplex number pairs for AH-1:

[AH-1]_(STS) =[a ₁ ,a ₂ ,a ₃ ,a ₄ , . . . ,a _(i) ,a _(i+1), . . . ].

Similarly for AH-2, AH-3 and AH-4:

[AH-2]STS=[−a ₂ *,a _(1*) *,−a ₄ *,a _(3*) , . . . −a _(i+1) *,a _(i)*,. . . ] where a _(i)* is the complex conjugate of a _(i),

[AH-3]_(STS) =[b ₁ ,b ₂ ,b ₃ ,b ₄ , . . . b _(i) ,b _(i+1), . . . ] and

[AH-4]_(STS) =[−b ₂ *,b _(1*) ,−b ₄ *,b _(3*) , . . . ,−b ₁₊₁ *,b _(i)*,. . . ].

For CD-C, applying a known space-time block coding algorithm with onespatial stream, [CD-C]_(SS), across two space-time streams, i.e. thesignals to be transmitted from the two access heads of the transmissionset {CD-C}_(Tx), yields the following for AH-6 and AH-7:

[AH-6]_(STS) =[c ₁ ,c ₂ ,c ₃ ,c ₄ , . . . c ₁ ,c _(i+1), . . . ] and

[AH-7]_(STS) =[−c ₂ *,c _(1*) ,−c ₄ *,c _(3*) , . . . −c _(i+1) *,c_(i)*, . . . ]

Each symbol of a space-time stream (e.g. a_(i)) is encoded as a pair ofsigned floating point numbers, one for the real part and one for theimaginary part. The radio controller 710 encapsulates the sequence offloating point number pairs for a given space-time stream inside apacket or frame that is addressed to the corresponding access head.Transmit timing information that is used by each access head toco-ordinate its transmissions with those of the other access heads inthe radio cluster 700 may also be included in the packet or frame.

In each access head, the stream of number pairs is extracted from thereceived packets or frames along with the transmit timing information.At the point in time dictated by the timing information, elements of thespace-time stream received from the radio controller 710 are modulatedby the access head onto OFDM carriers and transmitted over a radio linkto target CDs via the antenna(s) attached to the access head. If anaccess head or virtual access head is equipped with more than oneantenna, the same stream is transmitted over all antennas thus providinglocal transmit micro diversity but with no addition space-time coding.

Receive Chain Operations (From a Client Device)

Operation of the receive chain will now be described with reference onceagain to FIG. 8. Based on network access requests that have previouslybeen sent by one or more CDs, the scheduler 615 residing in the radiocontroller 610 selects a set of one or more CDs that will be allowed totransmit information within a given uplink transmission opportunity. Thescheduler 615 also determines which of the performance-enhancingtechniques may be applied to the signals received at the access headsfrom the CDs during this transmission opportunity. The scheduler 615provides coding instructions to each of the CDs via the access heads620,670 in a grant that authorizes the CDs to transmit its information.A spatial stream for each of the authorised CDs scheduled forcontemporaneous transmission along with streams from other authorisedCDs is processed by each respective CD to produce a space-time stream.The space-time stream is then modulated onto the OFDM carriers andtransmitted over the radio link by each respective CD.

The signal is received by the antenna 628 and processed by the OFDMdemodulator 624. Once demodulated the received symbols are forwarded bythe access head 620 to the radio controller. If more than one accesshead is allocated to receive the signal from the CD, the other accessheads perform similar activities to 620.

The spatial streams originally transmitted by each of the CDs arerecovered by the radio controller 610 from the received symbols usingits knowledge of pilot and training symbols associated with each of thespatial streams, the channel estimation information provided by eachaccess head, and known space-time decoding and de-multiplexingalgorithms in space-time decoder 642.

A reverse mapping from the detected modulation scheme (e.g. 64QAM) isthen applied at the digital demodulator 644 to each of the spatialstreams to reconstruct the bit stream corresponding to a PHY protocoldata unit (PPDU). In some implementations, descrambling and forwarderror correction is used to extract a MAC protocol data unit (MPDU) 648from the PPDU at the channel decoder 646.

If forward error correction fails to produce an error-free MPDU from atleast one of the spatial streams forwarded by the various access heads,known diversity combining techniques may be used by the radio controller610 to combine input from all of the streams related to a given CD in anattempt to reconstruct an error-free MPDU. If an error-free MPDU isreceived or reconstructed, it is passed to a higher-layer MAC functionfor processing; otherwise, error recovery procedures beyond the scope ofthis invention are initiated.

A particular example of receiving a transmission will be described withreferences to FIGS. 9 and 10. The radio controller 710 assumes that theserving set for each of the CDs is the same as that used for itsdownlink transmissions in FIG. 9. Referring to FIG. 10, the assumedserving sets will be:

{CD-A} _(˜serving)={AH-1,AH-2,AH-3}

{CD-B} _(˜serving)={AH-2,AH-3,AH-4}

{CD-C} _(˜serving)={AH-6,AH-7}

Because the assumed serving sets for CD-A and CD-B overlap, the radiocontroller assumes that their reception set will be:

{CD-A+CD-B} _(˜Rx)={AH-1,AH-2,AH-3,AH-4}

The radio controller 710 instructs CD-A and CD-B (and CD-C) regardingmultiple issues including, but not limited to, when to begintransmission, which space-time coding sequence each CD should use toencode their pilot symbols and the cyclic shift that each should applyto their training symbols. For example, the spatial stream sequence forCD-A will be:

[CD-A] _(SS) =[a ₁ ,a ₂ ,a ₃ ,a ₄ , . . . a _(i) ,a _(i+1), . . . ]

with pilot symbols:

[CD-A] _(PS) =[p _(A1) ,p _(A2) ,p _(A3) ,p _(A4) , . . . ,p _(Ai) ,p_(Ai+1), . . . ].

Similarly, the sequence for CD-B will be:

[CD-B] _(SS) =[b ₁ ,b ₂ ,b ₃ ,b ₄ , . . . b _(i) ,b _(i+1), . . . ]

with pilot symbols:

[CD-B] _(PS) =[p _(B1) ,p _(B2) ,p _(B3) ,p _(B4) , . . . ,p _(Bi) ,p_(Bi+1), . . . ].

The assumed reception set for CD-C does not overlap with those of CD-Aand CD-B, i.e.:

{CD-C} _(˜Rx)={CD-C} _(˜serving)={AH-6,AH-7}.

The (non-interfering) spatial stream sequence for CD-C will be:

[CD-C] _(SS) =[c ₁ ,c ₂ ,c ₃ ,c ₄ , . . . c _(i) ,c _(i+1), . . . ]

with pilot symbols dictated by the radio controller to be:

[CD-C] _(PS) =[p _(C1) ,p _(C2) ,p _(C3) ,p _(C4) , . . . ,p _(Ci) ,p_(Ci+1), . . . ].

When CD-B transmits, it is determined that the actual reception set forCD-B includes AH-5, but does not include AH-2. Also, it is determinedthat the actual reception set for CD-C includes AH-5. This change in theactual reception set for CD-B and CD-C as opposed to the assumedreception set may be due to movement of the devices or to a change inthe propagation environment caused by obstructers or reflectors, thatmay or may not be transient. Therefore, the reception set for CD-B turnsout to be:

{CD-B} _(Rx)={AH-3,AH-4,AH-5}={CD-B} _(˜serving)−{AH-2}+{AH-5}.

Similarly:

{CD-A} _(Rx)={AH-1,AH-2,AH-3}={CD-A} _(˜serving) and

{CD-C} _(Rx)={AH-5,AH-6,AH-7}={CD-C} _(˜serving)+{AH-5}.

The spatially multiplexed streams are demodulated from the received OFDMcarriers by each of the access heads and converted by each AH into asequence of complex symbols. At AH-1, the sequence is:

[AH-1]_(STS)=[1₁,1₂,1₃,1₄, . . . ,1_(i)1_(i+1), . . . ].

Similarly for AH-2 . . . AH-7:

[AH-2]_(STS)=[2₁,2₂,2₃,2₄,2_(i),2_(i+1), . . . ]

. . . .

[AH-7]_(STS)=[7₁,7₂,7₃,7₄, . . . 7_(i),7_(i+1), . . . ]

Each symbol of a space-time stream (e.g. 2_(i)) is encoded as a pair ofsigned floating point numbers, one for the real part and one for theimaginary part. Each access head encapsulates the sequence of floatingpoint number pairs for its received space-time stream inside a packet orframe that is addressed to the radio controller. Included in the packetor frame may be timing information indicating when the stream wasreceived by the AH and channel estimation information derived by the AHfrom the received pilot and training symbols.

The radio controller 710 extracts the stream of number pairs from thepackets or frames received from each of the access heads and uses thetiming information in those packets or frames to time-align the symbolsprovided by each of the access heads. The spatial streams originallytransmitted by each of the CDs are recovered by the radio controller 710from the received symbols using its knowledge of the pilot and trainingsymbols associated with each of the spatial streams, the channelestimation information provided by each access head, and knownspace-time decoding and de-multiplexing algorithms. Thus:

[1]_(STS) →[A] _(SS)

[2]_(STS) →[A] _(SS)

[3]_(STS) →[A] _(SS) +[B] _(SS)

[4]_(STS) →[B] _(SS)

[5]_(STS) →[B] _(SS) +[C] _(SS)

[6]_(STS) →[C] _(SS)

[7]_(STS) →[C] _(SS)

A reverse mapping from the detected modulation scheme (e.g. 64QAM) isthen applied to each of the spatial streams to reconstruct the bitstream corresponding to a PHY protocol data unit (PPDU). In someimplementations, descrambling and forward error correction is used toextract a MAC protocol data unit (MPDU) from the PPDU.

In some embodiments, the radio controller 710 updates the serving setfor each CD with the list of access heads reporting a spatial stream forthat CD:

{CD-A} _(serving)={CD-A} _(Rx)={AH-1,AH-2,AH-3}

{CD-B} _(serving)={CD-B} _(Rx)={AH-3,AH-4,AH-5}

{CD-C} _(serving)={CD-C} _(Rx)={AH-5,AH-6,AH-7}

and uses these sets for subsequent downlink transmission opportunities.

Numerous modifications and variations of the present invention arepossible in light of the above teachings. It is therefore to beunderstood that within the scope of the appended claims, the inventionmay be practiced otherwise than as specifically described herein.

1-20. (canceled)
 21. A method for use in a multiple input multipleoutput (MIMO)-based wireless network, the method comprising: performingat least some physical (PHY) layer functionality in a centralizedmanner; and performing at least some PHY layer functionality in adistributed manner, wherein the at least some PHY layer functionalitythat is performed in the distributed manner is (1) distinct from the atleast some physical PHY layer functionality performed in the centralizedmanner, and (2) comprises performing analog modulation/demodulation. 22.The method according to claim 21, wherein performing at least some PHYlayer functionality in a centralized manner comprises at least one of:performing channel coding/decoding; performing digitalmodulation/demodulation; and performing constellationmapping/de-mapping.
 23. The method according to claim 21, furthercomprising: in a centralized manner, performing at least some mediaaccess control (MAC) layer functionality.
 24. The method according toclaim 21, wherein performing at least some PHY layer functionality in acentralized manner comprises performing MIMO space-time coding/decoding.25. The method according to claim 21, further comprising, in acentralized manner, performing at least one of: scheduling oftransmissions; and selecting at least one technique from a set oftechniques for processing a transmit/receive signal.
 26. The methodaccording to claim 21, wherein performing analog modulation/demodulationcomprises performing orthogonal frequency divisional modulation (OFDM)modulation/demodulation.
 27. The method according to claim 21, furthercomprising in the centralized manner, providing synchronization signalsto control timing of wireless transmissions.
 28. The method according toclaim 21, wherein performing at least some PHY layer functionality inthe centralized manner comprises performing processing of a transmitsignal according to at least one of the following techniques: a)macro-transmit diversity; b) spatial reuse; and c) spatial multiplexing.29. The method according to claim 21, wherein performing at least somePHY layer functionality in the centralized manner comprises performingprocessing of a receive signal according to at least one of thefollowing techniques: a) macro-receive diversity; b) spatial reuse; andc) spatial multiplexing.
 30. The method according to claim 21, whereinperforming at least some PHY layer functionality in the centralizedmanner is performed by a radio controller and performing at least somePHY layer functionality in a distributed manner is performed by aplurality of remote access heads, each remote access head having acoverage pattern, the method further comprising: providing an overallcoverage pattern formed collectively by the coverage patterns of each ofthe plurality of remote access heads that is configured for one or moreof: tailoring wireless coverage to a localized area provided by theplurality of remote access heads to a desired size and shape; providingmore uniform Signal and Interference and Noise Ratio (SINR) across acontainment region; minimizing undesirable signal leakage to areasoutside of the containment region; and dynamically selecting a subset ofthe plurality of remote access heads during each transmission/receiveopportunity in order to minimize frame error rate and/or to maximizethroughput.
 31. A multiple input multiple output (MIMO)-based wirelessnetwork system comprising at least one radio cluster, the radio clustercomprising: a radio controller configured to perform at least somephysical (PHY) layer functionality in a centralized manner; and aplurality of remote access heads configured to be connected to the radiocontroller and wherein each remote access head is configured to performat least some PHY layer functionality distinct from that which the radiocontroller is configured to perform, the plurality of remote accessheads collectively providing coverage to a localized area; wherein theat least some PHY layer functionality performed by one or more of theplurality of remote access heads comprises analogmodulation/demodulation.
 32. The system according to claim 31, whereinthe at least some PHY layer functionality that the radio controller isconfigured to perform comprises at least channel coding/decoding ordigital modulation/demodulation.
 33. The system according to claim 31,wherein the at least some PHY layer functionality that the radiocontroller is configured to perform comprises constellationmapping/de-mapping.
 34. The system according to claim 31, wherein theanalog modulation/demodulation comprises orthogonal frequency divisionalmodulation (OFDM) modulation/demodulation.
 35. The system according toclaim 31, wherein the radio controller communicates with the pluralityof remote access heads using wired or unwired network links wherein:wired network link protocols include one or more of Ethernet,Infiniband, frame relay, and Asynchronous Transfer Mode (ATM); andunwired network link protocols include one or more of IEEE 802.11, IEEE802.16, Ultra Wide Band (UWB), and point-to-point microwave.
 36. Thesystem according to claim 31, wherein the radio controller is configuredto provide synchronization signals to the plurality of remote accessheads to control the timing of wireless transmissions, and the at leastsome PHY layer functionality performed by the radio controller comprisesMIMO space-time coding/decoding.
 37. The system according to claim 31,wherein for communications transmitted to or received from the pluralityof remote access heads, the radio controller is configured to performprocessing of a transmission or received signal according to at leastone of the following techniques: a) macro-transmit diversity; b)macro-receive diversity; c) spatial reuse for one or both of a transmitsignal and a receive signal; d) spatial multiplexing for one or both ofa transmit signal and a receive signal; and e) some combination thereof.38. The system according to claim 31, wherein for a given radio cluster,each remote access head of the plurality of remote access heads isconnected to the radio controller: directly via a point-to-pointconnection; or indirectly via a fan-out arrangement through one or moreother remote access heads.
 39. The system according to claim 31, whereinthe at least one radio cluster operates as a single frequency network.40. The system according to claim 31, further comprising a network linkbetween the radio controller and each of the plurality of remote accessheads, the network link configured to provide one or more of: facilitiesto transmit packets between the radio controller and each remote accesshead of the plurality of remote access heads; synchronization signals tobe transmitted by the radio controller to each remote access head of theplurality of remote access heads; and electrical power to be provided bythe radio controller to at least one remote access head of the pluralityof remote access heads.